It is known in the art that thermoplastic polymers can be loaded with inert fillers, cast into films, and thereafter stretched to form oriented thermoplastic films. While this statement is generally true, it must be realized that the particular components employed and the specific process parameters employed, particularly when control is difficult, can result in significantly different end products or contribute to the success or failure of obtaining a desired result. As an example, U.S. Pat. No. 4,118,438, discloses the use of some materials similar to those contemplated by the present invention, however, the object of U.S. Pat. No. 4,118,438 is diametrically opposed to the object of the present invention. U.S. Pat. No. 4,118,438 is concerned with a transparent polypropylene film containing fine particles of an incompatible polymer dispersed therein. The film disclosed exhibits surface projections caused by the dispersed particles and the patentees maintain that this gives the transparent film non-blocking characteristics. In U.S. Pat. Nos. 3,697,367 and 3,795,720, there is disclosed a process for preparing an uniaxially oriented mixed polymer system. The resulting material has utility as a paper substitute and can be formed into fibers for making sheet paper.
U.S. Pat. No. 4,377,616, the contents of which are incorporated by reference in their entirety, discloses an opaque biaxially oriented polymer film structure comprising a thermoplastic polymer matrix film core layer possessing numerous voids, a substantial number of which contain at least one spherical void-initiating particle, and transparent, void-free thermoplastic skin layers adhering to the surface of the core layer. The unique structure of the core layer imparts a much higher degree of opacity, possibly due to the effects of light scattering, than that possible from the use of an opacifying pigment alone. In U.S. Pat. No. 4,377,616, the film is prepared by melting a mixture of a major proportion of a film forming polymer such as polypropylene and a minor proportion of an incompatible polymer which has a higher melting point, at a temperature sufficient to melt the incompatible polymer and to dispense it in the film forming polymer, extruding the mixture into a film and biaxially orienting the film. The dispersed incompatible polymer provides sites for the formation of voids surrounding the polymer particles. These voids provide opacity and give the film an attractive pearlescent sheen.
Transparent, void-free thermoplastic skin layers are adhered to the surface of the core layer of the film of U.S. Pat. No. 4,377,616 to yield smooth film surfaces, free of the surface irregularities or surface projections of the core material. Without such skin layers being provided, U.S. Pat. No. 4,377,616, teaches that the lustrous appearance of the satin finish would be materially lessened. However, the use transparent, void-free thermoplastic skin layers to encapsulate the core layer can yield a film with somewhat undesirable end use characteristics. For example, when such films are utilized in certain label making operations, a skin layer surface may, through frictional contact, adhere to an excessive degree to the label processing equipment, resulting in labels of poor quality and/or equipment shut-down. Although the equipment can often be adjusted to minimize such problems, films which would provide a greater degree of operability would clearly be desirable. Another problem sometimes encountered in the end-use processing of the known opaque multilayer film structures employing transparent, void-free thermoplastic skin layers is that often such film structures are difficult to cut. Additionally, from an appearance standpoint, rather than provide a transparent skin layer, it is sometimes desirable to provide a skin layer of increased opacity.
Therefore, what is needed is a film structure of high opacity which provides improved cutting characteristics and an improved range of operability, while maintaining high strength and stiffness.